The invention relates to disconnect or cut-out switching systems for the start or auxiliary winding of a single phase AC induction motor.
A single phase AC induction motor typically includes a squirrel cage rotor and two stator windings, namely a main winding and a start or auxiliary winding. The main winding is connected across an AC power source and normally has low resistance and high inductance. The start or auxiliary winding is physically displaced from the main winding and has high resistance and low inductance. In a split phase type AC motor, the physical displacement of the motor stator windings together with the electrical phase displacement produced by the different resistance and inductance values generates a rotating magnetic field sufficient to provide a low level starting torque. Other types of single phase AC induction motors for higher torque applications include capacitor start type motors and capacitor start run type motors. In these types of motors, a capacitor is connected in series with the start winding to provide higher starting torque. In each of the motor types, after the motor has accelerated to 75-80% of its synchronous speed, a switch opens and the start winding is disconnected from the AC power source.
Various types of disconnection systems are known, including time delay type. A timer is started at initial energization of the motor. When the timer times out after a predetermined set timing interval, a switch is turned OFF to disconnect the start winding from the AC power source.
The present invention provides an improved and simplified motor starting circuit with time delay, type cut-out of the start winding. A start switch in series with the start winding controls energization and de-energization of the start winding. A turn-on switch controls turn-on of the start switch. A timing circuit controls turn-off of the start switch. The turn-on switch is connected in parallel across the start switch and not across the start winding, such that the start switch and the turn-on switch each see the same phase. This is significant because the turn-on switch and the start switch will thus turn on at the same time, without phase delay therebetween, which in turn maximizes energization of the start winding. In contrast, if the turn-on switch is connected across the AC line it will turn on at a different time than the start switch because of the phase difference therebetween due to the inductance of the start winding in series with the start switch, and the capacitance of the start capacitor if present. The turn-on switch supplies gate current to the start switch which is in phase with the AC line voltage but is not in phase with the voltage across the start switch due to the noted inductance and capacitance. This results in chopping or nibbling of the AC waveform applied to the start winding, which in turn reduces starting torque.
The present invention eliminates nibbling of the AC waveform applied to the start winding. In the present invention, the turn-on switch supplies gate to the start switch which is in phase with the voltage across the start switch to provide in phase turn-on of the latter such that a smooth continuous AC waveform is applied to the start winding to maximize energization thereof, without nibbling. This maximizes starting torque.
The invention also enables the addition of a simple minimum part content restart circuit without changing or redesigning the existing starting and cutout circuitry. The restart circuit provides for reconnection of the start winding to the AC source when motor speed drops below a given cut-in speed.